Reassessing the photochemical production of methanol from peroxy radical self and cross reactions using the STOCHEM-CRI global chemistry and transport model

doi:10.1016/j.atmosenv.2014.09.056

Atmospheric Environment

Volume 99, December 2014, Pages 77-84

https://doi.org/10.1016/j.atmosenv.2014.09.056 Get rights and content

Highlights

•
Photochemical production contributes significantly to the global CH₃OH sources.
•
The atmospheric life-time of CH₃OH is found to be 6.1 days.
•
The reaction of CH₃O₂ with OH could be a significant source of CH₃OH.

Abstract

Methanol (CH₃OH) is an oxygenated volatile organic compound (VOC) and one of the most abundant species present in the troposphere. The mass of CH₃OH in the atmospheric reservoir, its annual mass flux from sources to sinks, and its global budget have been investigated using STOCHEM-CRI, a global three-dimensional chemistry transport model. Our study shows that the global burden of methanol is 5 Tg. The atmospheric life-time of CH₃OH is found to be 6.1 days which falls within the range of previous modelling studies. The impact of peroxy radicals on the photochemical production of CH₃OH has been studied and suggests that NMVOCs (non-methane Volatile Organic Compounds) are an important source of both peroxy radicals and CH₃OH. The photochemical production routes of CH₃OH are found to be 48 Tg/yr, which are higher than the previous studies and contributes significantly to the total global methanol sources of 287 Tg/yr. An additional CH₃OH production of 8.2 Tg/yr from the reaction of methyl peroxy radicals (CH₃O₂) with hydroxyl radicals (OH) could be a significant additional source of CH₃OH particularly over the tropical oceans which would lead to a revision of the global sources and life cycle of CH₃OH.

Introduction

Methanol (CH₃OH) is one of the most abundant Volatile Organic Compounds (VOC) with concentrations of 1–10 ppbv in the continental boundary layer (Singh et al., 1995, Heikes et al., 2002). CH₃OH is transported globally (Singh et al., 2001) and accounts for a minor role in tropospheric oxidant photochemistry (Fehsenfeld et al., 1992, Kelly et al., 1994, Monod et al., 2000). CH₃OH oxidation is a non-negligible source of formaldehyde and carbon monoxide and leads to production of tropospheric ozone (Tie et al., 2003). However, CH₃OH sources and sinks are poorly quantified, with an estimated global source strength ranging from 120 to 340 Tg/yr and total sinks ranging from 40 to 270 Tg/yr (Jacob et al., 2005, Millet et al., 2008). There are large regional differences in the atmospheric mixing ratios of CH₃OH as a result of the wide variation in the source strengths from biogenic and anthropogenic contributions.

The loss due to OH oxidation is the main sink for atmospheric CH₃OH. The other minor removal processes including dry deposition, wet removal and oceanic up-take are poorly constrained. The overall atmospheric lifetime for CH₃OH with respect to the gross oceanic uptake, OH, and deposition is approximately 5–10 days (Jacob et al., 2005, Millet et al., 2008, Stavrakou et al., 2011). The resultant atmospheric lifetime implies that CH₃OH distribution is affected by surface emission and chemistry as well as by atmospheric transport. Direct emission sources of CH₃OH include living plants, plant decay, anthropogenic production, and biomass burning. CH₃OH is produced in most land plants during cell wall growth (Fall, 2003). The biogenic emissions are supposed to constitute the largest fraction of the global source of CH₃OH (Stavrakou et al., 2011). Photochemical production is another important source of CH₃OH, produced by the reaction of methyl peroxy radicals with itself and other higher organic peroxy radicals (RO₂) (e.g. Lightfoot et al., 1992, Tyndall et al., 2001), which provide a diffuse source most importantly in the remote atmosphere (e.g. Lewis et al., 2005).CH₃O₂ + CH₃O₂ → CH₃OH + HCHO + O₂CH₃O₂ + CH₃O₂ → CH₃O + CH₃O + O₂CH₃O₂ + RO₂ → CH₃OH + R_–HO + O₂CH₃O₂ + RO₂ → ROH + HCHO + O₂

Jacob et al. (2005) found that reaction (1a) contributed 85% of the photochemical source and the remaining 15% was contributed by reaction (2a), where RO₂ radicals were predominantly derived from biogenic isoprene, for CH₃OH production and was confined largely to the continental boundary layer (CBL). CH₃OH mixing ratios predicted by their global chemical transport model for the CBL were significantly smaller than measured values-suggesting a large missing source of CH₃OH in the CBL (Jacob et al., 2005). Thus, reassessing the photochemical production of CH₃OH from RO₂ may improve global estimates of the CH₃OH budget.

The atmospheric source of CH₃OH from reaction (1a) and (2a) were estimated in previous studies (Singh et al., 2000, Heikes et al., 2002, Galbally and Kirstine, 2002, Tie et al., 2003, von Kuhlmann et al., 2003a, von Kuhlmann et al., 2003b, Jacob et al., 2005, Millet et al., 2008, Wells et al., 2014) using different global tropospheric chemistry models and was found to be in the range of 18–38 Tg/yr. The variation of the estimates for different studies are as a result of the differences in the abundances of NO and HO₂ (which can act as dominant sinks of CH₃O₂), the CH₃O₂ reaction rate coefficients, the yield of CH₃OH from reaction (1a) and (2a), and the amount of higher peroxy radicals RO₂ (especially isoprene and terpene derived peroxy radicals). Jacob et al. (2005) found large discrepancies between observations from the PEM-Tropics B aircraft mission over the South Pacific with their modelling results, which they attributed to an under prediction of the photochemical source.

Most of the observations of atmospheric CH₃OH concentrations consist of short-term records, with very limited spatial and temporal coverage, so large uncertainties exist in the magnitude and distribution of global CH₃OH (Heikes et al., 2002, Jacob et al., 2005). Quantification of CH₃OH distributions and its global budget is necessary before accounting for its role in tropospheric oxidant photochemistry. In this study, the global distribution of CH₃OH has been produced using the STOCHEM-CRI global 3-dimensional chemistry transport model (Utembe et al., 2010, Archibald et al., 2010, Percival et al., 2013). We also incorporated the yields of CH₃OH from the reaction of methyl peroxy radicals (CH₃O₂) with hydroxyl radicals (OH) in the model that reduce the discrepancies between measurements and models highlighted over the oceans by Jacob et al. (2005).

Section snippets

Global Chemistry Transport Model STOCHEM-CRI

STOCHEM used in this study, is a global 3-dimensional Chemistry Transport Model (CTM) in which 50,000 constant mass air parcels are advected using a Lagrangian approach allowing the chemistry and transport processes to be uncoupled. STOCHEM is an ‘offline’ model with the transport and radiation codes driven by archived meteorological data, generated by the UKMO Unified Model (UM) at a climate resolution of 1.25° longitude × 0.83° latitude × 12 vertical levels (Johns et al., 1997). A detailed

Results and discussion

Table 3 summarizes the global model budget of CH₃OH. The global source (287 Tg/yr) of CH₃OH includes contributions from direct emission sources (83%) and photochemical production (17%), consistent with the results of Jacob et al. (2005). The photochemical production of CH₃OH is dominated by the reaction of CH₃O₂ with itself. The global sink is dominated by OH oxidation (76%), with another contribution from dry deposition (24%). In this model study, 48 Tg/yr of CH₃OH is produced photochemically

Conclusion

In this paper, we used STOCHEM-CRI, a global three-dimensional chemistry transport model to capture the global distribution and seasonal cycle of CH₃OH. CH₃OH has relatively long lifetime for a VOC of 6.1 days, thus a large amount of CH₃OH can potentially be transported from the continental boundary layer into the free troposphere where they have an impact on the formation of photochemical oxidants. The seasonal cycle of photochemical production of CH₃OH is controlled to a large extent by the

Acknowledgements

We thank NERC, GWR (Studentship for ATA), EPSRC (studentship for MCC) and EUROCHLOR under whose auspices various elements of this work were carried out.

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¹: Now at the Met Office, FitzRoy Road, Exeter, Devon EX1 3PB, UK.

²: Now at the School of Earth Sciences, University of Melbourne, Parkville, VIC 3010, Australia.

³: Now at the NCAS-Climate and the Centre for Atmospheric Science, University of Cambridge, Cambridge, UK.

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Reassessing the photochemical production of methanol from peroxy radical self and cross reactions using the STOCHEM-CRI global chemistry and transport model

Highlights

Abstract

Introduction

Section snippets

Global Chemistry Transport Model STOCHEM-CRI

Results and discussion

Conclusion

Acknowledgements

Chem. Phys. Lett.

Atmos. Environ.

Atmos. Environ.

Atmos. Environ. A Gen. Top.

Atmos. Environ.

Atmos. Environ.

Atmos. Environ.

Atmos. Environ.

On the importance of the reaction between OH and RO2 radicals

Atmos. Sci. Lett.

Impacts of mechanistic changes on HOx formation and recycling in the oxidation of isoprene

Atmos. Chem. Phys.

Peroxy radical and related trace gas measurements in the boundary layer above the Atlantic Ocean

J. Geophys. Res.

Methanol from TES global observations: retrieval algorithm and seasonal and spatial variability

Atmos. Chem. Phys.

Tropospheric ozone in a global-scale three-dimensional Lagrangian model and its response to NOx emission controls

J. Atmos. Chem.

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Chem. Rev.

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On the importance of the reaction between OH and RO₂ radicals

Impacts of mechanistic changes on HO_x formation and recycling in the oxidation of isoprene

Tropospheric ozone in a global-scale three-dimensional Lagrangian model and its response to NO_x emission controls